It varies depending on the duration of the two Dominated effects being applied. The book divides effect durations up into conditional or sustained durations, but for this purpose, we need to divide them differently:
- The creature is Dominated until the start/end of someone's next turn, or until the end of the encounter. This generally includes sustain effects. I will call this kind of duration time-based.
- The creature is Dominated (save ends). I'll call these save-based durations.
If both Dominated effects are time-based...
Whichever Dominated effect has the longest duration wins. From Overlapping Durations, from the Rules Compendium p227:
When a creature is subject to identical effects that end at different times, it ignores all but the effect that has the most time remaining.
So, all shorter Dominated effects are ignored. The effects are still there, technically, but doing nothing.
If there are multiple Dominated effects tied for the longest duration, they're probably all active (it's not clear). Proceed to the section about stacking.
If both effects are save-based...
Whichever effect came first wins. Or the character can choose which one wins. I'm not sure. From Identical Effects that a Save Can End, RC p228:
If a character is subject to identical effects that a save can end, ignore all but one of those effects. For instance, if the character is dazed (save ends) and then is attacked and again becomes dazed (save ends), ignore the second effect, since it is identical to the first one.
However, this may be just talking about making saving throws to end those effects. Either way, again, both effects are still there, technically. If you interpret this to only refer to saving throws, continue to the section about stacking.
However, there's a special case here:
If either is a multi-part effect...
Multi-part effects are those which say, for instance: "target is Blinded and Deafened (save ends both)." Normally, two blinded (save ends) effects will not overlap, and you can save from either to save from both, as we just covered. But a Blinded (save ends) effect will overlap with a Blinded and Deafened (save ends), meaning you'll have to save from both individually to be free of them.
If you have a Dominated (save ends) effect and Dominated and Something Else (save ends both) effect, both are active and neither can be ignored. See the section about stacking.
If there's a mix...
They stack. Continue.
If they're stacking...
You have a time-based and save-based effect, or two different save-based effects, or two time-based ones tying. In this case, they're both present and active. There's nothing to say which one wins. Neither gets removed. Two people are dominating you simultaneously. Both should be able to pick an action for you to take on your own turn.
As long as only one of your dominators wants you to take a standard action, things are OK, and both can make you take actions normally. If one makes you take a standard action and the other makes you take a move or minor action, you have enough action economy for that. Same for minor actions, or two move actions.
However... you only ever have one standard action, so the two dominators might have to mentally battle it out for control. Work out how this should work with your GM.
Note: Dominated no longer includes being Dazed.
The above wouldn't work by the version of Dominated in the first Player's Handbook, which states the Dominated person is Dazed, and thus would only ever get one action per turn. Under that version, the dominators would always have to battle it out over a single action.
However, somewhere along the way, Dominated got changed. I'm not sure when, but it was released with its new description in Player's Handbook 3, and then in the Rules Compendium. Unfortunately, it's not in PHB1's errata. Nowadays, Dominated doesn't mean you're dazed. It just means:
- You can't take actions voluntarily, you grant combat advantage, and you can't flank.
- Your dominator chooses an action for you on your turn.
This is basically the same as being Dazed whilst Dominated if you only have one dominator, but when multiple dominators come along, it makes all the difference in the world.